The field of the invention pertains to apparatuses and methods for generating VLF radio signals for data transmission. In more particularity, the invention resides in the field of digital techniques for data transmission having the purposes of (1) providing suitable signals for conducting performance tests of VLF digital data receive systems and, (2) providing suitable transmission sequences for training VLF receive system operators. In still greater particularity, the field of the invention concerns a digital record/reproduce system utilizing a built in digital modulator which provides all commonly used forms of VLF signal modulation during playback.
VLF radio signals having appropriate digital data trains and proper modulation are required for testing and training purposes relative to VLF receive systems. Previously, a colocated military VLF transmitter system has been used to provide the test or training signals. This test or training configuration ties up expensive military VLF transmitter equipment which is in short supply. Thus, there is a current need in the state of the art for a reliable, low cost, effective system for providing modulated VLF radio signals for test and training operations which has the capability for varying the modulation and the information content of the transmitted information while being compact and easily transportable to VLF testing facilities or training sites. Since the present invention provides such a system, a great savings in cost can be achieved.